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UK Broadcast Transmission |
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THE TRANSMISSION GALLERY
| Photos by the late Peter Kirby, and then given to Jill Rolfe | Page last updated: 2020-07-14 |
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| Crowborough, the high power medium wave transmitter site, was originally conceived and set up by the Diplomatic and Wireless Service, and was later designated as Her Majesty's Government Communications Centre (HMGCC), and finally Communication Engineering Department (CED). Built in the darkest days of the Second World War, it was used variously by Government and the BBC during that conflict, and then continued to be brokered to the BBC for transmitting the European Services. The site was closed in 1982 when the final transfer of operations to Orfordness was made. The purchase and installation, in 1941/42, of Aspidistra, the 600 kW medium wave transmitter procured to help the war effort with various forms of propaganda, is well known and documented (links at the bottom of the page). The transmitter consisted of three units, each of nominally 170 kW, which could be combined and over-run to produce 600 kW, and could also offer the possibility of "instant wavelength changes", where one unit was prepared and made ready to go on a new frequency while the other two carried on on the old frequency. The change to the new frequency could then be actioned in an instant, and the two units on the old frequency could then be retuned and powered up to restore the full whack on the new frequency. What the system could not do is to transmit on two frequencies at once, something that helps us a little in unpicking some of the post war deployments. The BBC were loathe to get too involved with a transmitter that at times broadcast propaganda, but while their own long wave and medium wave transmitter site at Ottringham (near Hull) was still under construction, some BBC European Service programmes were broadcast from Crowborough on a regular BBC frequency, namely 804 kHz, starting on 8th November 1942. Provision was made to synchronise Aspi with the other transmitters on 804 kHz (140 kW at Brookmans Park and 150 kW at Moorside Edge) when necessary. Pawley tells us that, along with the extensive use of Ottringham, by May 1945 Crowborough was broadcasting 20 hours a day of BBC European Service programmes; in July 1945 it changed frequency to 1122 kHz, with one day a week off for maintenance (Ottringham provided the back-up). The medium wave antenna consisted of three 350 ft guyed lattice masts in an arrow-head formation facing roughly east, the front mast was the only one that was driven, with the main ATU directly under it. The other two masts had ATUs as well but were tuned in a parasitic way, and not driven with RF from Aspi 1. The substantially-constructed 36 inch faced triangular masts were only lightly stayed with just two levels, the first at 130 ft and the second at 260 ft; the section above the 260 ft stay was therefore a cantilever construction, which was unusual for its time. There was an obstruction light at the top. This phased delta arrangement covered Germany well and then on into the Eastern Bloc countries, effectively 70 deg to 120 deg. |
| With the arrival of the Copenhagen Plan in March 1950, Crowborough moved to 1340 kHz whilst Ottringham took 1295 kHz. However, coverage wasn't satisfactory by day (Pawley) and from 22nd October 1950 Crowborough started using 647 kHz (464 m, the Third Programme allocation) during the daytime; in that era the Third didn't start until the early evening so the frequency was available, and was used for BBC programmes in French, Flemish, and Dutch. Meanwhile 224 m/1340 kHz was used extensively in the early mornings and late afternoon/evenings for English and more French programmes. Although Aspi 1 was capable of 600 kW, Copenhagen allowed only 150 kW on either 647 or 1340 kHz, so it's likely that only one unit at a time was ever in use. Ottringham closed in 1953 and 1295 kHz was transferred to the transmitter at Norden, on the German/Dutch border, which had been used by Hitler to broadcast Lord Haw Haw to Britain during the war. However, in April 1962 the lease of the Norden transmitter was ended, and the opportunity was taken to move 1295 kHz to Crowborough, still with only 150 kW. (1340 kHz was moved to a 20 kW reserve transmitter at Brookmans Park, where it presumably had minimal continental coverage at that power, but the object was to prevent squatters on the frequency while Lisnagarvey was retuned from 1151 kHz to 1340 kHz, a change that meant the loss of a frequency to the European Services but the ability, finally, to split the Northeast region and allow Northern Ireland to have its own regional programmes; Lisnagarvey took over 1340 kHz in January 1963). Starting in 1964, the newly-launched Music Programme (forerunner of daytime Radio 3) began to nibble at the "down-time" of 647 kHz at Daventry, and as a result the use of 647 kHz during the day at Crowborough had to cease. Whilst the war time configuration had been designed to blast power into Germany, it wasn't so suitable for many areas of France, and with the loss of daytime use of 647 kHz, with its very favourable ground wave propagation, it seems that reception of Crowborough for the French transmissions was not ideal on 232 m. As a result, the "French Modification" was put in place: this involved suspending an insulated triatic from the driven mast, with a "dropper" (consisting of 7 wires in a triangle with 6 ft faces) placed a quarter of a wavelength from the mast to act as a director, also a quarter of a wavelength high at 232 m. It was tuned against earth with its own ATU and a dedicated earth mat installed. The bearing of the dropper from the mast was orientated towards central France. The loss of 647 kHz daytime use was somewhat further mitigated in March 1967, when Crowborough started using 809 kHz (the Scottish Home Service frequency) for the lunchtime French. This - for propagation reasons - was only possible in the middle of the day if intolerable interference was to be avoided with the service north of the border, but it represented a useful improvement, although it's not clear how the masts were reconfigured to cope with the lower daytime frequency. On 4th September 1968, probably as a result of the brutal Soviet repression of the Prague Spring in Czechoslovakia, it was decided to restore full power working on 1295 kHz, and the full (normal running) 500 kW was back in use. Presumably it was felt by the late sixties that the fact that many countries in Europe, some friendly, some anything but, were flouting the allocations and powers of Copenhagen, required the BBC European Services to speak with a more powerful voice, in order to counter the Communist threat. |
| At the end of the sixties, other improvements and changes were in the pipeline. The BBC were under pressure to surrender some medium-wave allocations for the arrival of Independent Local Radio, which was to be set up under the auspices of the IBA. Reading the IBA summary of the period, it seems that there was considerable frustration on their side, as plans were changed and frequencies to be surrendered came and went, seemingly the IBA were expecting use of 1340 kHz and partial use of 1546 kHz, but the release of 1340 kHz was abandoned "for various reasons". Eventually it was 1151 kHz that was given over for exclusive use by the IBA, but in amongst all this turmoil the BBC and FCO decided that they wanted to restore the "twin wavelength use" for the European services, and one wonders whether this wasn't at least in part a Corporation ploy to save an additional frequency from the clutches of the commercial sector. It seems also possible that some of the to-ing and fro-ing may have resulted from various iterations of the DF (Deferred Facility) planning process, Crowborough being designated as a major DF site. It might be added that there were some late-night tests carried out by Aspi 1 on 1457 kHz in 1970 after closedown, but Andy Matheson remembers from that time that Aspi 1 was less than happy working on such a high frequency, becoming very hot with high circulating currents. Whether these tests were a trial of night-time propagation on 1457 kHz as a possible second frequency for the European Services, or whether they were connected to DF tests, is unclear; what they did do is to upset reception for many enthusiasts of Radio Geronimo, an alternative music station that had hired time late at night at weekends on Radio Monte Carlo's medium wave transmitter, using the next door frequency of 1466 kHz. Wherever the truth might lie over the frequency planning for the introduction of Independent Local Radio and the re-organisation of BBC medium wave services, the upshot was that from September 1972 Crowborough was allocated a second MW frequency, the old Midland Home Service wavelength of 276 m (1088 kHz). As a result it was necessary to have a second antenna available, and conveniently the decision had already been taken earlier to build a new six element Yagi for 1295 kHz; Pawley reports that this was completed in November 1970 and gave some 7 dB of gain in the forward direction, considerably improving the signal in north-west Germany. One wonders whether the "French Modification" to the original arrow-head antenna (referred to earlier) had in fact compromised the signal to the east (particularly in northern Germany) by slewing some of the power southwards. A prototype 180 kW Doherty transmitter was built and we assume that that was used on 1088 kHz from September 1972, as the BBC wouldn't have wanted to lose the use of the full 500 kW delivered by Aspi 1 for the Eastern European Services on 1295 kHz, which were already using the new 6 element Yagi. Subsequently however two more Dohertys were built, each of 250 kW, and paralleled to give a second 500 kW transmitter combo. As it was these Dohertys that were subsequently moved, a few years later and one at a time, to Orfordness to continue the 1295 kHz service, we can probably assume that it was Aspi 1 that eventually transmitted the 1088 kHz service from the original-but-French-modified trio of masts, and that it was the new Dohertys 500 kW combo that were fed into the 1295 kHz six-element Yagi, but it's possible that the transmitters and aerials could be swapped around to suit operational requirements. Andy Matheson relates that there were problems to overcome with the low feed impedance of the driven tower on the six element Yagi. By the time of the wavelength changes of November 1978 the 1295 kHz service (becoming 1296 kHz on 23rd November 1978) was already safely installed at Orfordness, having moved during 1977/78; on the actual day of the wavelength changes Aspi 1, who had stayed behind in her natural home, took up what was to be her final (but familiar) allocation of 648 kHz, where she held the fort for another four years while the five tower array and new 600 kW AEG Telefunken ORF1 transmitter were being built at Orfordness. Aspi 1 finally ceased service on 28th September 1982, closed down appropriately enough by Harold Robin, the man who had been responsible for the purchase of Aspi 1 in the first place. Commentary by Martin Watkins. I am indebted to many people who contributed to this page: Jill Rolfe who kindly allowed us to use the photos taken by the late Peter Kirby, to Dave Porter G4OYX who contacted several colleagues for information, and to David Smith G8IDL and Andy Matheson G3ZYP who responded to the call. |
Daventry | Orfordness | Woofferton
Media Network Wartime Deception Part-1
Article on Crowborough by Jill Rolfe
Media Network Wartime Deception Part-2
Farewell to 648 kHz
Tricks of the Trade: Aspidistra and OSE5
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